Despite important variations in regional family systems, little research has been done to assess the effects of these differences on fertility and thus on families’ economic status. Even less attention has been paid to the effects of deviating from these regionally embedded norms in terms of network compositions. People's social networks may not conform to the region's view of the ideal family, while this could have important implications for their fertility behaviour. To fill this knowledge gap, this paper aims to answer two questions: to what extent do family systems shape family size, and to what extent do deviations from regional family system norms in terms of social network composition result in differences in completed fertility? To answer these questions, we use the first two waves of the ‘Survey of Health, Aging and Retirement’ and derive indicators describing regional family systems and people's social networks. We test the influence of these covariates on the completed fertility of cohorts born between 1920 and 1960 in 13 European countries. Our results show that family system norms, and deviations from them in terms of specific social networks, play an important role in determining family size.